technical handbook for the paddy rice postharvest industry ... · pdf fileconveyor corp.) 48...

In the paddy postharvest system, paddy is moved, transported, or conveyed from

place to place. Traditionally, these have been hand operations. After harvest the

paddy is placed in gunny bags and transported several times through storage and

processing before the milled rice finally reaches the consumer. Paddy is often

handled too much, resulting in high handling costs and excessive losses.

More and more paddy is being handled by mechanical conveyors. Some convey-

ors replace hand labor; others supplement it or enable the same labor to move or

handle more paddy. Different types of conveyors are used. Screw and belt conveyors

move paddy horizontally or up small inclines. In some cases, chain and vibrating

conveyors are used. Bucket elevators are most common for lifting paddy vertically,

but occasionally inclined screw conveyors are used.

Paddy is a highly abrasive material and causes excessive and rapid wear on screw,

chain, and pneumatic conveyors. For this reason, bucket elevators and belt convey-

ors (both using rubber-covered belts) are preferred. They wear longer and are thus

usually the most economical. Vibrating conveyors have been used in certain parboil-

ing systems — mainly to permit excess water from the parboiling tanks to drain off

before the paddy enters the drying system.

When bulk paddy is handled, mechanical conveyors instead of hand labor are

required. In Sri Lanka in 1978, the conveying equipment for a 3,000-ton bulk store

cost $17,420. It replaced labor, which cost $2,630 a year. Therefore, the investment

cost of the mechanical conveyors was paid off in less than 7 years. Although there are

many other factors to consider, this illustrates the cost of mechanical conveyors vs

labor.

This chapter deals with the more popular and economical types of paddy convey-

ors. They include bucket elevators, belt and screw conveyors, and associated equip-

ment such as grain valves and spouting.

BUCKET ELEVATORS

General description

A bucket elevator consists of buckets attached to a chain or belt that revolves around

a bottom pulley (allowing the buckets to fill with paddy) and a top pulley where the

buckets discharge their paddy. The vertical lift may be a few meters to more than

50 m. Capacity may vary from 2 to 4 t/hour to as much as 25, 50, or even 100 t/hour.

A typical bucket elevator with details is shown in Figure 3.1.

Chapter 3CONVEYING

46 POSTHARVEST INDUSTRY HANDBOOK

3.1. Typical bucket eleva-

tor.

3.2. Elevators discharging

paddy: A, centrifugal; B,gravity; C, direct gravity.

3.3. Loading paddy into the boot of an elevator.

47 CONVEYING

TypesBucket elevators are available in several designs to handle many products. They are

classified according to the type of discharge used and are identified as centrifugal,

positive (gravity), and continuous (direct gravity). The three types are shown in

Figure 3.2. The centrifugal discharge type is most commonly used with grains. It is

designed and engineered to conform with general practice in handling grain. Head

and boot shafts are provided with roller bearings. Takeups are generally screw-type

except on tall high-capacity units where gravity-type take-ups are more common.

Buckets are usually made of steel or plastic and are bolted onto the belt. Casings or

legs are also made of steel, are welded or bolted together, and are dusttight. The

curved hood is designed for proper centrifugal discharge of the paddy grain. The

boot can be loaded from the front or back or both (Fig. 3.3). In larger, high-capacity

installations the head section is often vented and connected to an aspiration system.

Bucket types and capacities

Buckets are made of different materials and come in different shapes and sizes,

depending on requirements. Figure 3.4 shows a typical bucket used with centrifugal

discharge elevators. The buckets are uniform, smooth, and proportioned for fast

filling and quick, clean discharge. Figure 3.5 shows the correct method of bolting the

bucket to the belt.

Dimensions and capacities of different buckets are given in Table 3.1. The

carrying capacity is based on the angle of repose of paddy, which is normally 36° (see

line x - x in Fig. 3-4.) Because of the difficulty of loading all buckets to 100% of rated

capacity and the desirability of having a small reserve capacity in the elevator,

designers calculate carrying capacity on the basis of buckets being filled to 85 to 90%

of rated capacity.

Bucket spacing or the minimum vertical spacing between bolt holes of elevator

buckets is also shown in Table 3.1. Buckets may be as far apart as the required

capacity permits. lnstalling buckets closer than the minimum will probably result in

reduced carrying capacity because they will not fill properly at the recommended belt

speed.

Elevator capacities

The bucket elevator's capacity in tons of paddy per hour depends on bucket size and

spacing and on belt speed. Speed is the first critical factor to consider. The speed of

3.4. Bucket used for cen-

trifugal discharge elevator. (Courtesy of Screw

Conveyor Corp.)


3.5. Correct method of bolting the bucket to the

belt.

the belt in meters per minute depends on the head pulley speed. The recommended

head pulley speed depends on the pulley diameter. A properly designed bucket

elevator driven at the correct speed will make a clean discharge directly into the

throat of the head liner ensuring only slight paddy damage and little or no back-

legging or downlegging. If the head pulley speed is too slow, the buckets spill the

paddy into the legs. Paddy breakage occurs when the paddy is tumbled within the

pulley and re-elevated, as shown in Figure 3.6(A).

The optimum speed is shown in Figure 3.6(B). The buckets fill and carry optimally

and discharge the paddy directly into the throat — no spillage, no breakage.

If the head pulley speed is too fast, paddy is damaged by rough and fast handling

and the buckets will not fill properly. The buckets lose all their holding and discharge

control (Fig. 3.6(C)). The result is inefficient operation as well as excessive breakage

and undue head wear of the elevator top.

For optimum centrifugal discharge, the speed of the head pulley is calculated by

where R is the radius of the wheel plus one-half the projection of the bucket in

meters. Experience has shown that for paddy and most lightweight grains, a more

Table 3.1. Dimensions and capacities of elevator buckets.

Bucket size Capacity (cm 3)

(mm) when filled to

Length Projection Depth in Figure 3.4 line x-x

Normalspacingon belt (mm)

76102127152178203229254279305305

647089

102114127152152152152178

647695

114127140159159159159184

142283

566850

1416198231153398368139645380

102102127152165178203203203203229

49 CONVEYING

3.6. Elevator discharge at

different bucket speeds:

A, too slow; B, optimum,

C, too fast.

satisfactory operational speed is 80 to 85% of the theoretical speed. Table 3.2 shows

the recommended elevator speeds for different pulleys.

Thus, elevator capacity may be calculated from 1) bucket capacity and recom-

mended spacing found in Table 3.1, and 2) belt speed found in Table 3.2 as follows:

Elevator capacity (m 3/h) = (bucket capacity in m 3 /1,000,000) × (number of

buckets per meter of belt) × (belt speed in

meters/ minute) × (60 minutes/ h);

Pulleydiameter

(cm)

Table 3.2. Recommended elevator speeds for different size head pulleys.

Pulley Average circum-

Head pulley rpm bucket

ference projection Cal- Recom-

(cm) (cm) culated mended

30 94 10 66 41 129 10 60 51 160 10 54 61 192 13 49 76 239 15 44 91 286 18 40

122 383 20 36

56514642313431

Recommendedbelt

speeda

(m/min)

536513808998

119

aBelt speed (m/min) = (3.1416) × (pulley diameter in meters) × (recommended rpm).


then using 576 kg/m3 for paddy and one metric ton as 1,000 kg:

Elevator capacity (t/h) = (elevator capacity in m3/h) × (576 kg/m3) ÷

For example, take a 0.41 m head pulley with 127 × 89 mm buckets on 127 mm

(1,000 kg/t).

spacing:

m3/h = (.00056 cm) 1000/127 (65 m/minute) (60) = 17.2 m3/h

t/h = (17.2) (576) ÷ 1000 = 10

Table 3.3 shows representative capacities for various head pulleys at various

rpm's.

Elevator head section

Elevator heads should be of the proper shape and size with smooth contours. Figure

3.7 illustrates many of the design features that should be considered. The discharge

side of the head should be shaped so that material thrown from the buckets will not

be deflected into the downleg. The throat should be considerably below the head

shaft to catch materials that are slow leaving the buckets. Head section dimensions

for different size head pulleys are shown in Figure 3.8.

Lagging on the elevator head pulley (Fig. 3.9) is needed in pulling heavy loads.

Proper lagging increases the coefficient of friction between the pulley and belt. On

tall legs a backstop device is recommended to prevent the belt from running

backwards when elevator cups are loaded and power is cut off. A simple mechanical

rachet device serves well as a backstop.

The strut board at a 45° angle under the head pulley (Fig. 3.8) prevents the

accumulation of paddy and dust.

The throat plate should be easily replaceable so that it can be changed after it

wears out. The head shaft must be heavy enough to resist bending and to provide the

required torque carrying capacity. It must stay level and properly lined up. Antifric-

tion bearings, properly lubricated, are recommended.

Table 3.3. Average capacities of certain elevators with different speeds and bucket sizes.

Head pulley diameter

(mm)

Bucket

Size Spacing

(mm) (mm)

Capacity (t/h) with head pulley speed of

56

mm mm rpm rpm

46 31 34 51

mm

42

mm

31

rpm

30 89 × 64 102 × 70

41 102 × 70 127 × 89 152 × 102

51 152 × 102 178 × 114 203 × 127

61 203 × 127 229 × 152 254 × 152

76 254 × 152 279 × 152 305 × 178

91 305 × 178 356 × 178

122 356 × 178 356 × 203

102102

102127152152165178178203203203203229229229229254

2.65.2–––––––––––––

–––

––––

14.221.828.3

–

––––––––––

– – – – – – – – – –

6.310.012.5

– – – –

– – – – – – – –

– – – – –

– – – – – – – – – – – 31.0 – – – – 42.7 – – – – 46.6 – – –

– 51.3 – –

– – 55.6 – – – – 72.2 – – – – – 79.4 – – – – 96.2 –

– – – – 117.2 – – 137.6

–

– –

51 CONVEYING

3.7. Elevator head show-

ing desired features: A,

sprinkler head and alarm;

B, strut slanted to down

leg; C, cleanout opening;

lagged head pulley; F,D, inspection door; E,

buckets. Belt should be

2.5 cm wider then buck-

ets, pulley should be 2

cm wider than belt.

3.8. Head section design

details and dlmensions.

Elevator boot section

Most bucket elevators provide in the boot section a belt take-up device to tighten the

belt as required and to train it so that it runs true and does not rub on either side of

the boot. A manual screw-type takeup is most often used (Fig. 3.10). On tall,

heavy-capacity legs an automatic take-up boot pulley is used. This provides the

correct belt tension at all times.

The offset leg-type elevator shown in Figure 3.11 uses a boot pulley smaller than


3.9. Pulley lagging.

3.10. Elevator boot sec-

tion with screw take-up

adjustment.

the head pulley, mainly to conserve space around the elevator boot. It should be no

smaller than two-thirds of the diameter of the head pulley.

Grain entry may be on either side of the boot (Fig. 3.11). However, when grain

enters on the downleg, additional power is required for the “dredging effect” of

pulling the buckets through the grain in the boot.

Cleanouts should always be included on both sides of the boot to permit fast and

easy cleaning. They are usually placed at an angle (Fig. 3.10 and 3.11) and should

slide easily.

Elevator legs

Elevator legs are constructed as all welded, bolted, or riveted units. Cross sections of

different types are shown in Figure 3.12. They are manufactured in standard lengths

of 2.4 m, but could be manufactured in any length desired. The economics of local

manufacturing cost should determine which type of leg construction to use. Some

manufacturers find it more economical to employ singlebox construction that

includes both legs, as shown in Figure 3.13.

Belts for bucket elevator

Four types of belts are used for bucket elevators and belt conveyors: 1) duck, 2)

balata, 3) stitched canvas, and 4) solid woven cotton. Any of these belts may be

treated with special preparations or covered with natural or synthetic rubber.

53 CONVEYING

8.11. Boot section design

details and dimensions.

3.12. Cross don of dif-

ferent types of elevator

legs.

The standard cotton duck belt differs from ordinary sail duck or canvas in that the

strength of the warp (lengthwise threads) is considerably greater than that of the weft

(crosswise threads). Duck for belts is ordinarily graded as 28 oz, 32 oz, etc.,

according to the weight of a piece 91 cm long in the warp and 107 cm wide.

Balata belts are made of waterproofed cotton duck belts held together by balata, a

tree gum which is stronger than rubber at ordinary temperature but not so elastic.

Stitched canvas belts are multi-ply duck belts whose plies have been stitched

together and made waterproof. Solid woven belts are woven to thickness in looms

and are not of multiple construction. They are used primarily for power

transmission.

Most conveyor and elevator belts are of folded-ply construction. Some belts are

made by building up layers of plies that are cut or woven to the width desired and are

called "plied" construction belts. Table 3.4 shows minimum plies used in elevators.

The leverage on the bucket heads, due to the digging action and the load, increases

with greater bucket projections so that more plies are required to keep the bolts from

pulling through the belt.

Belt selection also depends on pulley diameters. Table 3.5 shows maximum plies

for standard pulley diameter.

The type of belt splice depends on the thickness of the belt and the severity of

service. For belts of five-ply thickness or less, the bolted clamp joint, the lap joint, or

the buttstrap joint may be used (Fig. 3.14).

For the clamp joint, belt ends must be bent outwards at right angles to form a

ridge that is then bolted between a bar clamp, On a lap joint splice, the lap extends a

distance of three to five buckets and is secured by the same bolts that hold the

buckets. (Use 20 mm bolts on four-ply belts, 25 mm for five- and six-ply, 32 mm for

3.13. Singlebox type elevator leg.

3.14. Splicing elevator

belts.


Table 3.4. Minimum plies for bucket projections

Grain Minimum plies when bucket projection is Belt

elevator 8 cm 10 cm 13 cm 15 cm 18 cm 20 cm fabric

5

5

28 oz or 32 oz

32 oz Low-speed 4 4 5 6 6

High-speed – – – 6 6

Table 3.5. Maximum belt plies vs diameter pulleys.

Head pulley Maximum Minimum foot

diam (cm) plies pulley diam (cm)

5161717691

56789

107 10 122 12

38465356667691

seven- and eight-ply.) This splice is not suitable for belts more than seven plies thick

because it is too stiff to pass tightly over the pulleys.

The butt-strap joint may be used on belts of eight or more plies. Place one bolt for

each 25 mm of belt width, 10 mm bolts for belts less than 10 plies, and 15 mm bolts

for those more than 10 plies.

Belt widths should be the bucket width, plus 25 mm. The pulley width should be

the belt width, plus 25 mm or more.

Accessories

For servicing the elevator head section, particularly the drive mechanism, a platform

for working is needed. Access to this platform is usually by ladder equipped with a

safety cage. A typical elevator with platform is shown in Figure 3.15. In some

installations, joint or common ladders are used for two or more elevators or other

machines.

55 CONVEYING

3.15. Elevator with plat-

form, ladder, and safety

cage. (Courtesy of Car-

dinal Division, LML

Corp.)

Power requirements

The theoretical horsepower (hp) requirements for bucket elevators may be obtained

from the equation:

where Q = capacity in kilograms per minute, H = lift in meters, and F = 1.5 for

elevators loaded on the down side of the boot, 1.2 for elevators loaded on the up side

of the boot. Actual horsepower requirements are 10 to 15% higher than this

theoretical value because of friction, power transmission, and drive losses. For

example, horsepower requirements for a bucket elevator with 1,600 bu/h of paddy

and a lift of 10.7 m loaded on the up side would be: (1,600 bu/h = 545 kg/minute)

Therefore the next larger standard size electric motor should be selected.


SCREW CONVEYORS

General infomation

Figure 3.16 shows a typical screw conveyor. It consists of a conveyor screw in a

trough supported by end and hanger bearings. The screw rotation pushes the grain

along the trough. The pitch (distance from the center of one thread to the center of

the next thread) of a standard conveyor screw is equal to its diameter. A 15-cm

diameter conveyor screw has a pitch of 15 cm. For each revolution of a standard

screw conveyor the paddy is advanced a distance equal to the pitch. The screw

conveyor is used to move paddy horizontally. It can also be used at any angle up to

90° from horizontal although there will be a corresponding reduction in capacity.

The helicoid screw (Fig. 3.17A) is a continuous one-piece helix shaped from a flat

strip of steel and attached to a pipe or shaft. Its thickness decreases from the inner

edge to the outer edge because of the strength necessary to form the helix (Fig.

3.17B). Smoothness of the helix is most important. Capacities and power require-

3.16. Typical screw con-

veyor.

3.17. Hellcoid conveyor

screw section.

57 CONVEYING

ments vary with segmented or welded sections.

Paddy is much more abrasive than most other grains and causes excessive wear on

the flights as well as the trough. To reduce wear, flights (helicoid section minus the shaft) may be fabricated from various materials such as stainless steel, monel, or

copper alloys. But because these materials are generally too expensive, a high-

carbon steel or other less expensive abrasive-resistant alloy is used.

A number of other conveyor flights are designed for special purposes. The ribbon

screw conveyors convey sticky materials. Another special type is a short-pitch

conveyor — pitch may be one-half of screw diameter or less — generally used in

feeders (Fig. 3.18). The short-pitch conveyor is used under a dump pit where full

loading of the screw is expected.

Screw conveyors may be designed for clockwise or counterclockwise rotation

without change in capacity. The screw conveyor carries the material as seen in Figure

3.19, on opposite sides (right-hand or left-hand). This characteristic may be consid-

ered in certain installations, such as feeding an elevator or machine.

Sizes and capacities

Screw conveyor components, in addition to the screw, include end bearings, hanger

bearings, inlet openings, and discharge openings (see Fig. 3.20 for details and general

dimensions). The dimensions of the helicoid screw are given in Figure 3.21.

Paddy assumes a cross section loading of 30% during operation of a screw

conveyor as shown in Figure 3.22. Based on this loading factor, screw diameter, and

rpm, the capacity for standard size screw conveyors is shown in Figure 3.22. For

screw conveyors of standard construction, the capacity chart should always be

followed for recommended maximum speeds. Speeds selected below the maximum

recommended are conservative. Speeds above that should be referred to the manu-

facturer before they are used.

From Figure 3.22 for example, a 15-cm conveyor at maximum speed of 120 rpm

has a capacity of 5.10 m 3/hour. With paddy of 576 kg/m 3, this is 2,937 kg or about 3.0 t/ hour.

(This is 39% of the theoretical calculated capacity based on the formula

Q= (D2 – d 2)/ 36.6 × P × rpm, where Q is in ft 3/ hour, D = screw diameter in inches,

d = shaft diameter in inches, and P = pitch in inches. Because of screw housing

clearance and the loading factor, the actual capacity is less than the theoretical

capacity.)

3.18. Short pitch conveyor section.

3.19. Conveyor screw sections: A and

B, right-hand; C and D, left-hand.

(top)

(right)


3.20. Schematic and di-

mensions for screw con-

veyor. (top)

3.21. Hellcoid flight con-

veyor screw and dimen-

sions (pitch = screw

diam). (bottom)

3.22. Capacity of screw

conveyors using helicoid

sections. Capacity and

power requirements are

different for segmented

and welded sections.

59 CONVEYING

Horsepower requirements may be determined by using the following formulas.

The determination does not consider power loss in drive equipment (belts, chains, or

gear reducers), imperfect alignment, or the power required for starting under load.

Additional power is therefore required for the average installation to overcome drive

losses and imperfect alignment.

(DS + QK)

1,000,000(1) H= L

Where L = overall length in feet

D = factor depending on type

S = speed in rpm

Q = quantity of paddy in

pounds per hour

K = material factor, for

paddy = 0.4

of bearings (Table 3.6)

(2)H×P

0.85hp=

Where P = 2 when H is less than 1

P = 1.5 when H is between 1

and 2


and 4


and 5

P = 1 when H is greater than

5 and 0.85 is estimated

efficiency of the drive

A sample problem:

paddy/ hour over a distance of 24 m.

Solution:

From Figure 3.22 (20 t/ h × 1,000 kg/ t) ÷ 576 kg/ m3 = 34.72 m3/ h), a 30-cm

screw at 92 rpm would be adequate. Then, assuming self-lubricating bronze

bearings from Table 3.6, D = 96, H = (24 × 3.281) × (171 × 150 + 44,080 × 0.4) ÷

1,000,000 = 3.41 then:

Determine conveyor size, speed, and horsepower requirements to move 20 t

3.41 × 1.25

0.85hp = =5.01

Table 3.6. “D” factors in computing horsepower for screw conveyors.

Conveyor

diam

(cm)

“D” factor for type of hanger bearings

Ball or roller

Wood, babbitt, bronze or moldedfabric

Self- lubricating

bronze

Whiteiron or

manganesesteel

7.5101523

25303540

10121832385578

106

152133546696

135186

24335496

114171255336

355080

130160250350480


The next standard size electric motor above 5 hp should be used. The following

specifications apply: Conveyor size: 30 cm

Conveyor speed: 92 rpm

Conveyor horsepower: 7.5

Screw conveyors can be operated in an inclined position with the flow of materials

upward. However, the allowable capacity rapidly decreases as the angle of inclina-

tion increases. A standard conveyor inclined 15 degrees will carry about 75% of its

rated horizontal capacity. At an incline of 25 degrees it will carry about 50% of its

rated horizontal capacity.

The additional horsepower required over the horizontal horsepower require-

ments is roughly 25% for a 15° inclined conveyor and 50% for a 25° inclined

conveyor. For a screw conveyor operated at an incline greater than 25°, a tubular

casing or a shrouded U trough should be used. It also becomes necessary at this angle

to use shorter-than-standard pitch flights.

Hangers and end bearings

The end thrust on a conveyor screw is against the direction of material flow. An end

thrust bearing assembly absorbs this force and prevents excessive wear of the

operating parts. A number of thrust arrangements are possible. One of the most

frequently used is an outside-type thrust bearing (Fig. 3.23). Preferably, the con-

veyor drive should be installed to drive through the end thrust because the shaft is

fixed in position and cannot “float” in the end bearing. However, the drive is often

installed on the feed end of the conveyor because of space or other limitations.

A standard-type hanger bearing used for screw conveyors designed for paddy is

illustrated in Figure 3.24. The 3.175 mm pipe tap provides a connection for a grease

fitting and is most often used with a pressure-type grease cup. Additional life can be

obtained by using ball bearings with dust seals in the hanger. Shields should be

placed on the upstream side of the bearing to protect it from grain pressure and wear.

Inlets and discharge openings

Generally, inlet openings may be cut into the conveyor trough cover wherever

needed. Figure 3.20 shows inlet spouts at two locations. Inlet openings should be

kept at a sufficient distance from hanger bearings to prevent clogging or choking at

3.23. Outside type thrust bearing for heavy duty service.

61 CONVEYING

3.24. Standard hanger

bearing.

3.25. Screw conveyor discharge openings and

spouts.

that point. For general use, the inlet opening is square and of the same dimensions as

the inside width of the trough. The opening may be flared or an inlet spout may be

designed to meet specific needs. Special side opening inlets can also be designed to

control the depth of material fed to the trough at that point.

Discharge spouts may be flared or made longer to meet special machinery needs.

A standard opening is square and equal to the inside width of the trough. Several

types of discharge openings and spouts are illustrated in Figure 3.25.

Troughs and covers

A variety of screw conveyor troughs exist. Two types common to paddy require-

ments are shown in Figure 3.26: the flanged type with flanged cover installed (A) and

the angle flanged type without cover (B). Most troughs for handling paddy are made

of high carbon steel or abrasive-resistaat alloys to withstand the severe wear.

Other types of trough covers are illustrated in Figure 3.27. The flat cover is used

indoors where waterproofing is not necessary. For most outdoor conveyors where


3.26. Screw conveyor

troughs. (top) 3.27. Trough covers and

clamps.

waterproofing is essential, the flanged-hip roof cover is used. Screw cover clamps are

most often used with both types of covers.

Drive arrangements

Because most screw conveyors are operated at relatively low speeds and electric

motors operate at relatively high speeds, a speed reducer is essential. Drives can be

direct coupled, or belt or chain connected as shown in Figure 3.28.

63 CONVEYING

3.28. Drive arrangements

for screw conveyors: A,

speed reducer mounted

on conveyor shaft, motor

mounted with V belt con-

nection to slde or top; B,

self-contained unit with

standard speed reducer

mounted on the shaft,

motor attached and driven

by V belt; C, gear motor

with built-in speed redu-

cer, chain drlve to screw

shaft,

3.29. Auger systems: A,portable; B, installed-in-

bin.

Portable and bin augers

The previous section on screw conveyors provides design data on heavy-duty,

continuous-operation screw conveyors, the type which would be used in a paddy

storage-processing plant.

In many small storage installations, however, it may be necessary to load and

unload bins only a few times per year. This type of operation does not require the

type of screw conveyors just described. A number of manufacturers produce special

screw conveyors for occasional use. They are generally known as augers. Figure 3.29

shows portable augers A used on an incline to move paddy from a tractor-trailer into

a dryer and from the dryer into a storage bin, and a horizontal unloading auger B

under the floor of a storage bin and a vertical auger to move paddy from the storage

bin to a truck or into a rice mill. These light-duty augers (fewer operating hours per

year) are housed in lightweight closed tubes instead of an open U trough.

Most often they are operated at higher rpm’s than those recommended for

heavy-duty screw conveyors shown in Figure 3.22. Thus they achieve higher capaci-

ties than small-diameter screw conveyors. Generally these augers are less expensive


3.30. Horizontal auger

with details: A, auger

housing, flighting, and

stubs; B, intermediate

flighting bearings; C, end

plate with bearing: D, re-

duction unit; E,drive unit.

(Courtesy of GT Augers)

3.31. Distributing and un-

loading augers.

than heavy-duty types and are more attractive to the operator who does not require

continuous heavy-duty operation. An example of a horizontal auger with details of

its components is shown in Figure 3.30. This type could be used either as an

overhead distributing auger or as a bottom unloading auger (Fig. 3.31). Capacities

and operating rpm's for different size augers are shown in Table 3.7.

Table 3.7. Capacities of light-duty augers.

Auger diam

(cm)

Capacity

(m3/h)

Operating

rpm

Minimum hp requirements for augers measuring

8.5 m long 12.8 m long 18.6 m long

15 35.4 450 3 520 42.5 218 5-7.5 7.520 70.8 370 – –

25 84.9 205 – 10-152s 106.2 260 – –

15

20

7.5

–

–

65 CONVEYING

When the auger is operated in a vertical position, the capacity is greatly reduced.

For example, when the 15-cm auger in Table 3.7 is operated vertically it has only 20

m3 /hour capacity at 620 rpm; the 20-cm auger, only 35.4 m3 / hour capacity at 620

rpm. Capacities vary with manufacturers.

Portable augers are designed with the same type trough and screw flight construc-

tion as the horizontal augers, but they need extra outside reinforcement because of

their long lengths (Fig. 3.32). They are adjustable in height or angle to meet the needs

of different size bins or dryers. They may be powered by electric motors or gasoline

engines, or be driven by a tractor power takeoff. They are available in 15-cm, 20-cm,

or 25-cm diameters. Their capacities vary considerably; for example, a 20-cm auger

operating at an angle of 20° may have a capacity of 70 m3 / hour. But the same size

auger operating at 45° may have a reduced capacity of only 50 m3 / hour.

Minimum electrical horsepower requirements could increase by 1/3 with wet

paddy if the same capacity in cubic meters per hour is maintained.

Other portable augers are available in smaller diameters, in varying lengths, and

for different operational needs.

BELT CONVEYORS

A belt conveyor (Fig. 3.33) is an endless belt operating between two pulleys with its

load supported on idlers. It may be flat for moving bags of paddy, or V-shaped for

3.32. Portable auger.

(Courtesy of GT Augers)

3.33. Typical belt con-

veyor.


moving bulk paddy. The belt conveyor consists of a belt, drive and end pulleys,

idlers, a drive and tension mechanism, and loading and discharge devices. Its

carrying capacity depends on the belt width, angle of trough, and belt speed.

Belt conveyors have a high mechanical efficiency because the load is carried on

antifriction bearings. Damage to paddy is virtually nil because there is no relative

motion between the paddy grains and the belt. Carrying capacity is high because

relatively high speeds are possible. Paddy can be conveyed a long distance. A

properly designed and maintained belt system has long service life and low operating

cost. The initial cost is high for short distance belts and relatively low for long

distance belts compared to other types of horizontal conveyors. For these reasons,

belt conveyors are widely used to move paddy in many installations. They range

from 30-100 cm in width, and may be up to several hundred meters in length.

The load cross section of a troughed belt is shown in Figure 3.34. Cross section

areas of loaded belts of various sizes are given in Table 3.8. A trough angle of 20° is

best suited for paddy and most other grains. Other common trough angles are 35°

and 45°. Paddy forms a surcharge angle ( A in Figure 3.34) of 20°. Other common

surcharge angles are 5° and 30°.

Belt inclination for paddy and most grains is limited to 15-17°. With inclines

larger than this, the grain begins to roll or slide back down the belt thus reducing its

effective carrying capacity.

To determine the required belt width, the following formula is used with Table

3.8:

Capacity (bu/ h) = (area of cross section in m2) × (speed

in m/ minute) × (60) × (28.25)

3.34. Cross section of

loaded belt: A is surcharge

angle.

Table 3.8. Cross-section area of loaded belt and maximum belt speeds.

Belt width

(cm)

Clear Total cross section Operation speeda

margin area (m 2) for 20° (mlmin)

(cm) surcharge angle Normal Maximum

30.535.640.645.750.861.016.2

4.14.34.64.85.15.66.4

.0072

.0089

.0122

.0161

.0204

.0308

.0504

616161767691

107

122122137137152183213

aBelt speed should be 91 m/min where a tripper is to be used, and 46-76 m/min

where a plow is to be used.

67 CONVEYING

Example: Determine belt width and speed to convey 1,200 bu of paddy/ hour. Using

the cross sections from Table 3.8, a 35.6 cm belt traveling at 81 m/ minute would give:

.0089 × 81 × 60 × 28.25 = 1,221 bu/ hour

and a 30.5 cm belt traveling at 99 m/minute would give:

.0072 × 99 × 60 × 28.25 = 1,208 bu/ hour.

In this example the 35.6-cm-wide belt at 81 m/ minute is adequate, unless a tripper is

to be used (minimum of 91 m/minute for tripper use). Then the 30.5-cm belt at 99

m/minute should be used.

The top idler spacing should be 1.5 m for belts up to 0.5 m wide and 1.4 m for belts

0.6-0.9 m wide. The return idler spacing for belts up to 0.9 m wide should not exceed

3 m. After belt speed in meters per minute has been determined, then rpm of the head

pulley shaft can be calculated with Table 3.9 as a guide.

The horsepower required for moving paddy by belt conveyor may be calculated

by the following formulas that are based on the lift, friction resistance of the belt and

the pulleys, and tripping device.

Belt speed A + B (3.281 L )hp 1 =

0.3048×

100

hp 2 = (t/ hour) × 0.48 + .01 L

100

lift t/ hour hp 3 = × 1.015 ×

0.3048 l000

Where L = belt length in meters, belt speed is in meters per minute, lift in meters, and

A and B are constants from Table 3.10.

Table 3.9. Revolutions per minute (rpm) of pulley shaft for various belt speeds

and pulley diameters.

Belt speed

(m/min)

Pulley shaft rpm when pulley diam is

50 m 60 cm 76 cm 90 cm 110 cm

3046611691

107122

20283848556575

16243241485565

1420

2532384551

11162227323843

9141824273236

Table 3.10. Constants for determining horsepower for belt conveyors.

Conveyor belt width (cm)

Constants

A B

Additional hp for tripper

3641

46506076

0.20 .00140 0.25 .00140 0.30 .00162 0.30 .00187 0.36 .00224 0.48 .00298

0.700.851.001.401.702.50


3.35. Cross section of belt

conveyor: A, drive pulley;

B, idler; C, tripper, D, feed

hopper; E, adjustable belt-

tensioning pulley. (below) 3.36. Belt loading arrange-

ment showing metal or wood frame with rubber

skirt and extra set of

idlers. (bottom)

The total horsepower required is the sum of the powers calculated from the three

equations, plus that required for the tripper from Table 3.10.

For example: A belt carrying 18 t/hour, 41 cm wide, traveling at 110 m/minute for a

distance of 90 m, with a 3-m lift:

110 0.25 + (.0014) (3.281) (90) hp 1 =

0.3048×

100= 2.39

.048 + (.10) (90)

100hp 2 = 18 ×

hp 3 = 3

0.3048

18× 1.015 ×

1000

= 0.25

= 0.18

Plus for the tripper (from Table 3.10) = 0.70

Total hp = 3.52

A cross section of a belt conveyor with its major parts is shown in Figure 3.35.

Note that the feed hopper is installed near the upper end of the belt. It has guides on

the side to prevent the paddy from splashing off as it feeds onto the belt. These guides

may be metal or wood, 60-90 cm long, installed slightly above the belt surface to

prevent wear on the belt. Details are shown in Figure 3.36.

In Figure 3.35, the belt moves along the top idlers, which are spaced close together

to carry the load. The tail pulley is adjustable to maintain the correct belt tension.

Paddy is discharged by a belt tripper, which is movable along the length of the belt

and incorporates a two-way discharge valve so that the paddy can be discharged on

69 CONVEYING

either side of the belt.

A standard set of top idlers is shown in Figure 3.37. The side idlers are positioned

at the 20° angle required for paddy. They are designed for periodic lubrication. The

center roller has an extended grease pipe on the side for easy access. Figure 3.38

shows dimensions of standard idlers. Tapered roller bearings with outer dust and

water seals are most commonly used.

Self-aligning idlers (Fig. 3.39) are used for training troughed belts. They automat-

ically correct belt misalignment due to off-center loading, uneven belt stretch,

misalignment of supports, or other common field working conditions. A self-

aligning idler replaces a standard idler every 15-30 m.

Self-aligning idlers should use the same rollers and bearings as the standard idlers.

Self-aligning camber idlers (Fig. 3.40) installed on the return belt help train the

loaded belt. The idlers are often overlooked in conveyor belt designs.

Paddy generally is discharged from the belt conveyor over the end pulley or at any

point along the conveyor by a scraper plow or a throw-off carriage known as a

tripper. The discharge over the end pulley of the belt is simple and does not require

any special mechanism. A common use of this type discharge is from a belt conveyor

into the boot of a bucket elevator.

The discharge scraper plow is a board placed at an angle (usually 30 to 49°) to the

longitudinal axis of the belt and fastened on a frame that can be raised or lowered

onto the belt as required. In the operating position, the plow rests on the belt,

pressing against it with a rubber strip fastened to the board. The plow can be used to

discharge paddy from either side of the belt. As the paddy grains traveling on the belt

come in contact with the plow, they are deflected to the side of the belt and

discharged. The discharge plow is not commonly used with paddy because small

paddy grains often slide under the plow and are not discharged at the desired

3.37. Standard idlers for

belt conveyors. (Courtesy of Continental Conveyor

Corp.)

3.38. Dimensions of stand-

ard idlers using 12.7 em

center rollers.

3.39. Self-aligning idlers.

(Courtesy of Continental

Conveyor Corp.)

location. Also, some grains are crushed or cracked between the plow and the belt

surface. If the plow is adjusted too close to the belt, excess belt wear results.

Trippers are available as hand-propelled, self-propelled, or automatic. The choice

depends on the particular installation, its capacity, and system operation. A simple

light-duty hand-propelled tripper and its dimensions are shown in Figure 3.41. Note

the direction of belt travel. As the belt passes over the top pulley, the paddy is

discharged immediately and spouted to the left or right as desired. The hand crank


3.40. Self-aligning cam-

ber return idlers. (Cour-

tesy of Continental Con-

veyor Corp.)

3.41. Schematic and di-

mensions of light-duty,

hand-propelled tripper

for various belt widths.

(Courtesy of Continental

Conveyor Corp.)

71 CONVEYING

with chain drive on the side permits the operator to move the tripper in either

direction along the belt. A tripper normally is installed on two I-beams running the

length of the belt conveyor and becomes part of the conveyor's frame.

Note the two-way flip-flop valve in Figure 3.41, which permits the grain to be

discharged on either side of the belt as desired. This is a simple arrangement used in

3.42. Vertical automatic

gravity belt tensioner.

(below)

3.43. Rubber lagged pul-

ley. (right)


most installations.

The tripper also has a locking device that keeps it from moving when the belt is

carrying paddy. The device is usually a vertical pin, dropped through a fixed opening

to keep the tripper in the locked position during operation.

For heavy-duty installations and when the tripper is to be moved often, a

self-propelled or automatic tripper is often preferred. The self-popelled tripper

involves an extra set of pulleys which are used to drive the tripper.

Automatic gravity take-ups are recommended for the longer belt conveyors to

properly maintain the required driving tension at the head pulley. Where the proper

tension has been obtained by this type of take-up, it will be maintained for the life of

the belt, independent of operating conditions. Figure 3.42 shows a vertical unit. The

take-up frame slides up and down on pipe guides. The minimum take-up pulley

diameter in inches equals four times the number of belt plies. Minimum bend pulley

diameter in inches equals three times the number of belt plies.

Figure 3.35 shows a horizontal, adjustable tail pulley for belt tension adjustment.

This may be a simple screw adjustment for short, low-capacity conveyors (the same

type used for belt tension in bucket elevators in Fig. 3.11). Or it may be similar to that

in Figure 3.42, where the weight is suspended over a set of pulleys for gravity control.

Pulleys with rubber lagging are recommended when additional traction between

belt and pulley is required and when pulleys are operated under wet conditions.

Lagging may be vulcanized to the pulley (Fig. 3.43) or bolted on (Fig. 3.9). Wing-

type pulleys are usually heavy-duty, all-welded construction. The sloping wing plates

automatically shed the material to each side of the pulley to prevent buildup on the

pulley face, which can cause considerable damage to the belt. Welded steel wing

pulleys (Fig. 3.44) are recommended for tail shafts of belt conveyors and boot shafts

of bucket elevators. For pulleys with diameter of 30-34 cm, 9 wings are provided;

40-cm pulleys have 10 wings; 44-60 cm have 12 wings; and 66-91 cm have 16 wings.

Belt conveyor drives can be of several designs; the choice depends on the economy

of available materials. The most common are the gear motor type directly connected

or connected with a chain drive. V-belts are used from motor to a countershaft or

connected with a chain drive. V-belts are also used from motor to a directly

connected speed reducer (Fig. 3.45).

73 CONVEYING

3.44. Wing-type pulley.

3.45. Belt conveyor drives

A, simple chain drive from

power source; B, direct

gear motor; C, gear motor

with chain reduction to

shaft; D, V-belt drive to

countershaft with gear to

head shaft; E, V-belt to

speed reducer (direct

connected); F, motor to

head shaft.

speed reducer coupled to

OTHER PADDY CONVEYORS

Several other type conveyors are occasionally used to move paddy. They include

shaker (vibrating), chain (drag), and pneumatic conveyors. The major disadvantage

of the shaker conveyor is its limited capacity. The major disadvantage of chain and

pneumatic conveyors is their short life due to the extreme abrasiveness of paddy


compared to other grains. Because these conveyors are seldom used for moving

paddy, their discussion is limited.

Shaker conveyor

Shaker (also called vibrating, oscillating, or grasshopper) conveyors move paddy in

a uniform, continuous flow by the upward and forward oscillating motion of a

continuous trough that is mounted on sturdy inclined reactor legs (Fig. 3.46). The

conveyor consists of a steel or wood trough mounted on flat spring, resilient support

legs with a positive action drive. The drive consists of a motor turning a shaft on

which an eccentric provides the oscillating action to the trough.

The shaker conveyor is designed for horizontal conveying. It is particularly suited

for moving wet paddy from the parboiling tanks to the dryer. In this case, the bottom

of the trough is perforated to permit excess water from the parboiled paddy to drain

before reaching the elevator leg.

The carrying capacity of the shaker is small. A trough 30 cm wide by 10 cm deep

would be limited to about 5 t/ hour. A 46- × 10-cm trough would have a capacity of

about 7.5 t/ hour. Horsepower requirements are low. A 5-t/ hour shaker conveyor

would be limited to a maximum length of 21 m and would require 1 hp.

Chain conveyor

Chain conveyors are inexpensive, slow, noisy, and mechanically inefficient. To move

paddy, scrapers or drags used with the chain operating in a closed container or

trough incur excessive wear due to the abrasive paddy. Figure 3.47 shows the cross

section of the conveyor trough and the normal movement of paddy as it is dragged

by the conveyor chain. Horsepower requirements for chain conveyors are more than

for belt or shaker conveyors of the same capacity. Because of the highly abrasive

nature of paddy, the expected life of the chain conveyor is considerably less than that

of the belt conveyor.

3.46. Shaker or vibrating-

type conveyor showing spring mounting and

power drive.

75 CONVEYING

3.47. Cross section of

chain-drag conveyor.

Pneumatic conveyor

Pneumatic conveyors move material in a closed-duct system by a high-velocity air

stream. The system uses a material feeder or collector, an air blower, ducts, and a

cyclone for collection or discharge. Figure 3.48 illustrates a common use of the

pneumatic system — unloading ships or railcars and conveying the grain into a

storage or another handling system.

The power requirements for a pneumatic conveyor are high. A larger problem is

the excessive wear on the equipment caused by the highly abrasive paddy. Therefore,

the pneumatic system is seldom used for moving paddy.

The pneumatic conveying system is most useful in handling less dense paddy husk

and other by-products such as bran and fine brokens. It is hard to beat for handling

husk and bran. Wear is minimized by the proper duct design, velocity considerations

(not too fast, not too slow), and matching the system to the requirements.

3.48. Typical pneumatic

conveying system.

3.49. Portable conveyor. (Courtesy of Cardinal

Division, LML Corp.)


Portable conveyor

Another type of portable loader or inclined elevator is shown in Figure 3.49. It is

chain driven with paddles attached to the chain, which operates in a metal trough. It

is used to move bulk paddy and gunny bags of paddy to higher sites. It may be 9-21 m

long and is most useful as a supplement to labor in stacking bags of paddy in large

storage facilities.

GRAIN VALVES AND SPOUTING

In most paddy-handling installations, grain valves and grain spouting are used with

bucket elevators and screw conveyors. Paddy moves by gravity through these grain

valves and spouting. Because the angle of repose for paddy is 36-37° all spouting is

installed at a minimum angle of 45°. Discharge valves are designed for this minimum

angle and as long as that angle is maintained, paddy should flow freely.

77 CONVEYING

Most spouting is straight steel pipe with flanged or unflanged ends, depending on

installation requirements. Spouting 15 cm in diameter is usually available in 14 or 12

ga; 20 cm is available in 10, 12, or 14 ga. Standard 22.5°, 45°, and 60° elbows are

available. Adjustable elbows that make installation much easier are also available.

For capacities of 1,500 bu/hour (30 t/hour) or less, 15-cm spouting and valves are

adequate. For capacities from 1,500 to 3,000 bu/hour (60 t), 20-cm spouting is used.

From 3,000 bu to 4,500 bu/hour (90 t), 25-cm spouting is used.

The impact of falling paddy is extremely abrasive and rapidly wears out spouting

and elbows. A grain trap such as that shown in Figure 3.50 is used where falling

paddy would cause excessive wear on the elbow. The abrasive action of the paddy is

absorbed by the trapped grain and saves wear on the elbow.

3.50. Grain trap used to

reduce wear on elbows.

3.51. Two-way valve with bucket-type gate.


Two- and three-way grain valves are the most common. They operate on either

the bucket-type gate as shown in Figure 3.51 or the flop gate as shown in Figure 3.52.

Spring tension enables the valve to open in either direction, The internal gate is

fabricated of abrasive-resistant steel. The valve may be operated by chain or cable.

Normal dimensions for these valves are given below:

Valve dimensions (cm)

A B C D E

15-cm spouting

20-cm spouting

21.6

26.7

25-cm spouting 31.8

15.2

20.3

25.4

38.1

45.7

54.6

31.7

36.8

40.6

50.8

57.2

61.0

Two- and three-way valves are also available with round inlets and outlets. They

come in different styles (Fig. 3.53) depending on the installation requirements. The

same general dimensions given for gates with square inlets and outlets apply.

Many installations require multiple distribution valves. An elevator in a drying

system may be connected to six tempering bins and one dryer. For more than three

outlets, distributors are used. Typical distributors are shown in Figure 3.54. Note

that the inside of each turns to make connections with the desired opening or spout.

Distributors can be made with many spouts and usually contain one spout as an

overflow. Dimensions of a typical distributor are shown in Figure 3.55.

3.52 Three-way valve with

flop gate. (top) 3.53. Two-way and three-

way valve arrangements.

(right)

79 CONVEYING

3.54. Five-way distributor.

(top)3.55. Schematic and di-

mensions for two sizes of

distributors. (upper right)

3.56. Schematic of eleva-

tor. A, total height; B,

head clearance; C, nor-

mally quoted effective ele-

vating height; D, headloss due to distributor or

valves;E, effective elevat-

ing height with distribu-

tor; F, hopper height up leg feed; G, hopper height

down leg feed. (lower

right)


The distributor is controlled by either cable or steel rod from an indicator at

ground level. Note the control rod at the bottom of the distributor in Figure 3.55.

Dimensions of grain valves and distributors are important in determining the

required elevator height. Because grain spouting from an elevator to a dryer or a bin

must be kept at 45° angle, the height of the valve or distributor is added to the height

of the elevator.

Figure 3.56 shows the effective elevator height as the total height minus 1) head

clearance, 2) head loss due to the distributor, and 3) hopper height. The normal

quoted effective elevating height is the total height minus only the head clearance

and the hopper height.

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